Solid ink or phase change ink printers conventionally use ink in a solid form, either as pellets or as ink sticks of colored cyan, yellow, magenta and black ink, that are inserted into feed channels through openings to the channels. Each of the openings may be constructed to accept sticks of only one particular configuration. Constructing the feed channel openings in this manner helps reduce the risk of an ink stick having a particular characteristic being inserted into the wrong channel. U.S. Pat. No. 5,734,402 for a Solid Ink Feed System, issued Mar. 31, 1998 to Rousseau et al.; and U.S. Pat. No. 5,861,903 for an Ink Feed System, issued Jan. 19, 1999 to Crawford et al. describe exemplary systems for delivering solid ink sticks into a phase change ink printer.
After the ink sticks are fed into their corresponding feed channels, they are urged by gravity or a mechanical actuator to a heater assembly of the printer. The heater assembly includes a heater that converts electrical energy into heat and a melt plate. The melt plate is typically formed from aluminum or other lightweight material in the shape of a plate or an open sided funnel. The heater is proximate to the melt plate to heat the melt plate to a temperature that melts an ink stick coming into contact with the melt plate. The melt plate may be tilted with respect to the solid ink channel so that as the solid ink impinging on the melt plate changes phase, it is directed to drip into the reservoir for that color. The ink stored in the reservoir continues to be heated while awaiting subsequent use.
Each reservoir of colored, liquid ink may be coupled to a print head through at least one conduit. The liquid ink is pulled from the reservoir as the print head demands ink for jetting onto a receiving medium or image drum. The print head elements, which are typically piezoelectric devices, receive the liquid ink and expel the ink onto an imaging surface as a controller selectively activates the elements with a driving voltage. Specifically, the liquid ink flows from the reservoirs through manifolds to be ejected from microscopic orifices by piezoelectric elements in the print head.
As throughput rates for liquid ink print heads increase, so does the need for delivering adequate amounts of liquid ink to the print head. One problem arising from higher throughput rates is increased sensitivity to resistance and pressures in the print head flow path. Restricted ink flow can limit or decrease imaging speed. In systems having filtration systems for filtering the liquid ink between the reservoir and a print head element, the flow may also change over time and become insufficient to draw liquid ink to the print head in sufficient amounts to provide the desired print quality.
One way of addressing the issue of flow resistance is to increase the filter area. The increased filter area decreases the pressure drop required to migrate a volume of ink through the filter. Increasing the filter area, however, also increases the cost of the printer as filtration material is often expensive. Moreover, the space for a larger filter may not be available as space in the vicinity of a print head of in a phase change printer is not always readily available.
Another way of overcoming flow resistance as well as increased volume demand with fast imaging is to pressurize the liquid ink to force the ink through a restrictive flow path. The pressure needs to be introduced after the ink has left the melt plate as melt plates do little to pressurize the fluid. The approach of introducing pressure, however, increases the complexity of the printing system, adds a pressure source and related components to the printer, and introduces another maintenance issue for the operational life of the printer.
Melt plates have been formed as tapered chambers and ink sticks are fed into a wide inlet of the tapered chamber. The walls of the tapered chamber are heated to a temperature that melts the solid ink sticks. The increased surface area of the chamber helps reduce the time required for melting an ink stick. The faster melt rate with the increased melt surface allows faster imaging.
One limitation of the tapered melt chambers is the additional opportunity for flow away from the chamber at points other than the intended exit point near the smaller portion of the tapered geometry. Consequently, the tapered chamber must be oriented to ensure gravity influences flow to the intended exit. As noted previously, space constraints may be rather restrictive in some phase change ink printers, which makes it difficult or impossible to configure the ink delivery system to rely on gravity flow control. Also, space above the print head may not be available for a melt chamber having an adequate length to width ratio to achieve the desired melt surface area.